1. Field of the Invention
The invention concerns a process for producing defined layers or layer systems of polymers or oligomers on any solid surface, with a controlled structure, wherein each layer is chemically deposited on the solid surface by means of a xe2x80x9clivingxe2x80x9d/controlled radical reaction. The invention further concerns solid surfaces provided with oligomer or polymer layers, as well as various compounds with an anchor group as well as a group or functionality from which polymerization growth occurs according to the ATRP mechanism. These types of compounds are referred to in the following as initiators.
The present invention concerns a process based upon this mechanism of xe2x80x9clivingxe2x80x9d/controlled radical reaction and polymerization for chemical modification of various solid surfaces. The solid substrate could be of any freely selected material, could be solid or porous, could be finely spread, could be of natural or synthetic origin, or exhibit a heterogeneous surface structure or heterogeneous surface composition. The physical-mechanical characteristics of the utilized solid substrate are not of consequence in the process, nor are hardness, ductility, deformability or surface roughness.
The term xe2x80x9csurfacexe2x80x9d herein is not limited to the outer surface. in the conventional sense, where it is generally understood to mean the boundary between the solid and the gas or liquid environment. The present invention is equally applicable to internal surfaces of a porous material. Beyond this, when using the inventive surface modifying materials, the term xe2x80x9csurfacexe2x80x9d is also intended to refer in general to various phase boundaries. Thus the surface could also be, for example, an interface between two different components of a composite material. Examples of this type include composites of a polymer matrix and an inorganic reinforcing agent, a polymer filled with a dyestuff, or a polymer-metal composite; generally also included are composites of a polymer matrix and a functional additive.
The surface characteristics of the solid substrate can be tailored by chemical modification to meet requirements. On the one hand, a desired quality can be imparted to the surface; on the other hand, the quality of the physical interactivity of the surface modified solid to other materials, the chemical reactivity, and the ability to form chemical bonds with other materials can be adjusted as required.
If layers or layer systems are applied to the surface, then in certain cases the characteristics of the original surface can be so changed, that the characteristics of the system as a whole will be influenced only by the coating. Thus it is possible, for example, to provide a composite material with mechanical stability by selecting a suitable carrier or backbone material, and on the other hand to adjust the desired mechanical, physical and/or chemical characteristics of the surface by using the layer system.
2. Description of the Related Art
In order to modify solid surfaces by application of polymers, various techniques could be employed. Processes are described in the literature, in which dissolved polymers are sprayed, spin-coated, dip-coated or applied according to the Langmuir-Blodgett-Technique (LB-films). The bonding of the polymers to the surface therewith almost exclusively is of an adhesive nature. The process parameters frequently are difficult to control when using these processes; in addition, particularly in the case of the Langmuir-Blodgett-Technique, this only can be employed on planar surfaces and substantially is limited to amphiphilic or rigid chain molecules.
Polymer molecules also can be bonded to the solid surface chemically, by forming a covalent chemical bond with the solid surface via mostly terminal groups of the polymer molecules (xe2x80x9cgrafting-toxe2x80x9d, for example via a condensation reaction). One disadvantage of this process is that the yield with this type of surface reaction, and therewith the graft density of the polymer molecules upon the surface, generally is not very high since previously bonded polymer molecules hinder the approach of subsequent molecules to the surface. Further, the process is limited to polymers with relatively low molar mass, since only with small molecules there is a sufficiently high probability that the functional group of the polymer molecule is within the reach of the bonding sites on the solid surface and thus a chemical reaction between the two would be possible.
In order to circumvent the disadvantages associated with xe2x80x9cgrafting-toxe2x80x9d-processes, in accordance with further developed processes the polymer reaction for forming the polymers is initiated or triggered directly on the solid surface (xe2x80x9cgrafting-fromxe2x80x9d) [J. Rxc3xche, xe2x80x9cMassgeschneiderte Oberflxc3xa4chenxe2x80x9d, Nachr. Chem. Tech. Lab. 42 (1994) 1237]. Therein, in the state of the art for polymerization reactions beginning with solid materials, the classical radical grafting reaction is generally described: for initiating the radical polymerization reaction conventional initiators are employed, that is, azo compounds, peroxide, among others. If one attaches this type of initiator covalently to the solid surface in order to thereby initiate the grafting reaction, this is associated with the following disadvantage: in symmetrical initiators such as for example azo-bis-isobutyro-nitrile (AiBN) or benzoyl peroxide (BPO) there is, after decomposition, one fragment covalently bonded to the solid as initiating radical, the second radical fragment remains unbound and can for its part initiate a polymerization reaction, which however does not occur on the solid surface, but rather unbound. In a polymerization solution with the above mentioned covalent initiators there is thus formed, besides the bound polymer, always also non-bound polymer.
This situation has lead thereto, that an alternative path has been sought using asymmetric initiators in which, after decomposition, the unbound radical fragment has essentially no reaction initiating effect. This is described in detail for example in the writings of Rxc3xche et al. [O. Prucker, J. Rxc3xche, Macromolecules 31, 592 (1998); O. Prucker, J. Rxc3xche, Macromolecules 31, 602 (1998)].
In addition to this, all until now conventionally initiated radical polymerization reactions are subjected to the classical kinetics of radical polymerization, that is, the graft branch length and the termination reactions can only be insufficiently controlled and the chain length is subjected to the typical chain length distribution of classical radical polymerization [s. Bruno Vollmert, Grundriss der Makromolekularen Chemie, Bd. I, E. Vollmert-Verlag, Karlsruhe, 1979]. Further, the chain ends of the graft branches are no longer reactive after the polymerization reaction, so that for example an additional polymerization for a second generation of polymer is not possible.
This disadvantage of radical polymerization was overcome in large part recently by a new process. If a radical polymerization reaction is carried out using a xe2x80x9clivingxe2x80x9d/controlled radical mechanism, then defined polymers can be produced, of which the chain length and polydispersity can be controlled substantially better than in the case of classical radical polymerization. Since the number of the chain terminations in this process is strongly reduced, the term xe2x80x9cstable free radical polymerizationxe2x80x9d (SFRP) is also employed. This process experienced a further refinement, as taught by K. Matyjaszewski et al., by the introduction of the concept of the xe2x80x9catom transfer radical polymerizationxe2x80x9d (ATRP) [K. Matyjaszewski, S. Coca, S. Gaynor, Y. Nakagawa, S. M. Jo, xe2x80x9cPreparation of Novel Homo- and Copolymers using Atom Transfer Radical Polymerizationxe2x80x9d, WO 98/01480]. xe2x80x9cLivingxe2x80x9d/controlled radical polymerizations, also in their refinement according to the ATRP-mechanism, have until now only been carried out in the liquid phase, with or without supplemental solvent.
Craig J. Hawker et al. describe in ACS Polym. Preprints [Div. Polym. Chem. (39), 626 (1998)] the synthesis and application of polymers with utilization of xe2x80x9clivingxe2x80x9d radical polymerization reactions. As initiators for the radical polymerization, compounds are employed which contain a nitroxide group. These compounds generally exhibit terminal trichlorosilyl groups which can be bound to surfaces of silica gel and silica wafers by chemical reaction.
Tsujii et al. in Macromolecules 1998, 31, 5934 describe controlled graft polymerization of methyl methacrylate on silicon oxide containing substrates by the combined use of the Langmuir-Blodgett-(LB)-Technique and the ATRP Technique (Atom Transfer Radical Polymerization). As initiator compound 2-(4-chlorosulfonylphenyl)-ethyltrimethoxysilane is used. This compound possesses a chlorosulfonyl group as initiator group for the xe2x80x9clivingxe2x80x9d/controlled polymerization. After depositing a monolayer of the above mentioned initiator, which had been compressed on a water/air interface by means of LB-Technique, on a silicon wafer, the xe2x80x9clivingxe2x80x9d/controlled radical polymerization of methyl methacrylate is carried out on this modified surface.
These processes according to the state of the art exhibit the following disadvantages: In the xe2x80x9cstable free radical polymerizationxe2x80x9d SFRP with use of nitroxides it often occurs that, due to the necessarily high temperatures of 120 to 130xc2x0 C., frequently simultaneously thermal polymerization reactions proceed, which do not initiate at the surface. Therewith substantial disadvantages are associated with the process for the xe2x80x9clivingxe2x80x9d/controlled radical grafting of solid surfaces, namely
a) the production of unbound polymer consumes monomer,
b) the growing bonded as well unbonded polymer chains compete for nitroxide and influence therewith the control of the growing chains,
c) unbonded polymer is present as reaction product, besides the polymer modified solid surface.
According to the publication of Tsujii et al., 2-(4-chlorosulfonylphenyl)-ethyltrimethoxysilane is employed as initiator compound. Chlorosulfonylphenyl groups are known to be highly reactive and in particular sensitive to hydrolysis, so that they are difficult to work with. Compounds which exhibit such groups, including the therewith modified surface, are unstable.
Further the LB-method described in this publication can only be applied to planar substrate surfaces, and this on limited surface areas and not, however, on solid surfaces of any size, shape and composition, as well as not on materials with open pores on their surfaces. The density of the molecules on the layer can only be incompletely influenced. The testing of the degree of polymerization of the grafted-on polymer molecules is only carried out indirectly, the grafted-on polymer molecules themselves are not involved therein. Further, it is not achieved that the chain ends are capable of further initiation.
This technical task is solved by a process for production of defined layers or layer systems of polymers or oligomers with controlled structure on any solid surface, wherein the layers are deposited chemically on the solid surface by means of xe2x80x9clivingxe2x80x9d/controlled radical reactions, by the following steps:
a) bonding compounds of the general formula
Axe2x80x94Lxe2x80x94Ixe2x80x83xe2x80x83(1)
xe2x80x83to the solid surface via the anchor group A, where A represents an anchor group, I is the initiating group for ATRP polymerization, and L is the linkage between A and I.
b) carrying out xe2x80x9clivingxe2x80x9d/controlled free radical polymerization according to the ATRP-mechanism by reacting the initiator group I bound to the solid surface with monomers, macromonomers or mixtures able to undergo free radical polymerization, which produces the polymer layer on the solid surface.
The invention concerns a process which employs xe2x80x9clivingxe2x80x9d/controlled radical polymerization for simultaneous and defined chemical modification of any solid surfaces. Therein oligomer or, as the case may be, polymer molecules are formed directly on the solid surface via a xe2x80x9clivingxe2x80x9d/controlled radical polymerization reaction. According to the state of the art, prior to the development of the present invention radical polymerization reactions initiated from solid surfaces were not controllable, or could only be controlled with difficulty, with respect to the chain length of the growing polymer chains. According to the invention, now radical reactions, which begin with appropriately chemically modified solid surfaces, can be carried out without termination, that is they can be controlled in a targeted manner. Therewith the graft branch length can be tailored with simultaneously maintaining a narrow chain length distribution; likewise it becomes easily possible to produce block copolymers as graft moieties.
The advantage of this inventive process is comprised therein, that the method of the xe2x80x9clivingxe2x80x9d/controlled radical polymerization can be employed upon any solid surface by using initiator groups which are easy to use and widely employable due to their stability, and which have the characteristic of making it possible to use the xe2x80x9clivingxe2x80x9d/controlled radical polymerization according to the ATRP-mechanism at temperatures below 120xc2x0 C., without however simultaneously suffering from thermal polymerization.